A Guidebook to Mechanism in Organic Chemistry

Thermal Fission of Benzoyl Peroxide

Even at room temperature, and particularly as the temperature is

raised, solutions of the peroxide are observed to liberate C0 2 due to:

O

II

Ph—C—O* Ph* + CO,

Thus, phenyl radicals will be present as well as benzoate radicals and

often in quite considerable concentration; this is, indeed, one of the

most useful sources of phenyl radicals. Production of the radicals in

solution, as is tMrtlormal practice, can lead to further complications.

Thus with benzene as solvent, the following initial reactions can, in

theory, take place:

O

Ph—C—O

Ph—C—b

II

o

2Ph—C—O' + Ph—H

/

I

\

-co,

Ph' + Ph—H I

\

Ph—CO,H+Ph*

f

Ph—CO,Ph+H-

Ph—Ph+H*

1

Ph—H-l»Ph-

...0)

...(ii)

...(hi)

...(iv)

Reaction (iv) will, of course, not be directly detectable, but would
serve to prolong the apparent life of phenyl radicals in the solution.
In fact, (i) is found to be the main reaction taking place. It should be
emphasised, however, that the above is only the first stage, for either
O

Ph—C—O or Ph can then attack the products derived from (i),
(ii), and (iii). Thus, further attack on diphenyl from (iii) by Ph* leads
to the formation of ter- and quater-phenyl, etc. It should, however,
be pointed out that reactions (ii) and (iii) are almost certainly not
direct displacements as shown, but proceed by addition followed by
removal of a hydrogen atom from tiie addition product by another
radical (cf p. 256): •—^

Ph*+H- :>+RaH